Under-reamer tool

ABSTRACT

The invention provides an under-reamer tool and method for creating a large diameter volume within a well bore. The under-reamer tool includes a mandrel for connection to a drill pipe and for supporting a torque arm assembly and hole initiator assembly. The hole initiator assembly, upon activation, creates a nominally larger diameter volume that triggers activation of the torque arm assembly thereby allowing the torque arm assembly to extend to cut away a cutting face thereby creating a significantly larger diameter volume within the wellbore as the under-reamer is rotated within the wellbore. Reverse circulation of a circulation medium in combination with the geometry of the cutting face provides effective removal of the cuttings from the cutting face.

FIELD OF THE INVENTION

[0001] The invention provides an under-reamer tool and method for creating a large diameter volume within a well bore. The under-reamer tool includes a mandrel for connection to a drill pipe and for supporting a torque arm assembly and hole initiator assembly. The hole initiator assembly, upon activation, creates a nominally larger diameter volume that triggers activation of the torque arm assembly thereby allowing the torque arm assembly to extend to cut away a cutting face thereby creating a significantly larger diameter volume within the wellbore as the under-reamer is rotated within the wellbore. Reverse circulation of a circulation medium in combination with the geometry of the cutting face provides effective removal of the cuttings from the cutting face.

BACKGROUND OF THE INVENTION

[0002] Under-reamers are used to enlarge cased or uncased wells to provide a larger diameter volume at depth. They are generally transported through the well on the bottom of a tubing string or drill pipe string to a location where the larger diameter volume is desired. At this location, the under-reamer is activated, and the larger diameter volume is reamed or cut out by the under-reamer tool as the drill pipe is advanced and rotated in the well.

[0003] In the past, various problems and limitations have existed in the design and operation of under-reamers particularly for creating larger diameter cavities. In particular, as the diameter of the under-reamed section becomes significantly larger relative to the diameter of the well bore, significant problems exist in designing systems that allow the cuttings to be efficiently removed from the created cavity. That is, as the cavity diameter increases, exponentially higher circulation speeds and hence volumes of a circulation medium (typically fluids) are required to properly lift the resulting cuttings up through the annulus of the well by the normal circulation path of down the drill string interior and up through the annulus between the well bore and the drill string.

[0004] In addition, there exists a problem of providing sufficient torque strength to long radius-cutting tools that are also capable of being collapsed. That is, under-reaming tools must be capable of being successfully run into the wellbore to the point where under-reaming is required, deployed for the under-reaming and subsequently collapsed again to enable retrieval of the tool through a narrow wellbore. As the length of the torque arm is increased, it becomes increasingly difficult to achieve both deployment and retrieval.

[0005] Thus, in view of the above problems, there has been a need for an under-reamer tool that can be positioned at any location in a cased or un-cased well bore whereby actuation of the tool enables the creation of a larger diameter cavity at that location. There has also been a need for an under-reamer tool that can cut through casing if required, to avoid a separate operation to first mill the casing before running an under-reaming tool into the well. In addition, there has been a need for an under-reamer tool that enables proper cleaning or removal of the debris from large diameter cutting faces through reverse circulation of the circulation medium (typically fluid) up through the system.

[0006] A review of the prior art reveals that a tool addressing these problems has not been developed.

[0007] For example, U.S. Pat. No. 2,169,502 (issued Aug. 15, 1939) discloses a well bore enlarging tool having cutting arms, torque arms and a main spring for biasing the tool into an open position. FIG. 1 shows the tool in the compressed position while FIG. 2 shows it in the expanded position. The tool is operated by placing the tool below any well bore casing which then cause the main spring to advance the cutting arms. Further pressure can then be applied to the cutting arms by applying hydraulic pressure against a piston 35. The cutting arms are collapsed to enable the tool to be drawn up-hole by dropping a “go-devil” down-hole that restricts fluid passage to the piston. This patent does not disclose the use of a hole initiator assembly having initiator fingers or a system that enables reverse circulation to effectively remove the cuttings required to create a large diameter cavity.

[0008] U.S. Pat. No. 2,450,223 (issued Sep. 28, 1948) discloses an under-reamer having longer cutter arms. This system uses a combination of a ball and cam-actuated cutting arms to initiate cutting. The system further requires that the system be resting on the bottom of the well relying upon the weight of the system to advance the cutting arms once actuated. Moreover, this patent does not teach reverse circulation.

[0009] Other prior systems include those disclosed in U.S. Pat. No. 2,882,019 (issued Apr. 14, 1959), and U.S. Pat. Nos. 2,754,089,2,755,070, 3,339,647, 3,554,305 and 3,568,784 which disclose various embodiments of hydraulically expandable rotary drill bits, which are expanded and contracted in response to fluid circulation and fluid propelled tool actuators.

SUMMARY OF THE INVENTION

[0010] In accordance with the invention, an under-reamer tool is provided that enables a large diameter cavity to be milled down hole. In addition, the invention provides a system in which reverse circulation of fluid is utilized to assist cutting and to efficiently remove cuttings up the drill string. Moreover, the design of the tool enables it to be easily positioned in a new or existing wellbore that is cased or uncased (including making a large diameter cavity at the bottom of a new or existing wellbore). The design of the tool also enables it to be easily retrieved from the wellbore after the large diameter cavity has been created over a particular interval.

[0011] More specifically, and in accordance with the invention, there is provided an under reamer tool for creating a large diameter cavity within a well bore comprising a mandrel for operative connection to a drill pipe, the mandrel operatively supporting a torque arm assembly and hole initiator assembly, the hole initiator assembly, upon activation, for creating a nominally larger diameter cavity thereby triggering activation of the torque arm assembly.

[0012] In a preferred embodiment, the mandrel and hole initiator assembly are bollow enabling reverse circulation of the well bore after activation of the hole initiator assembly.

[0013] In various other and more specific embodiments, the torque arm assembly includes a cutter arm carrier in sliding engagement with the mandrel. The torque arm assembly may also include at least one cutter arm and at least one torque arm, wherein each cutter arm is pivotally connected to the cutter arm carrier and to a torque arm and wherein each torque arm is pivotally connected to the mandrel. The torque arm assembly may also include an activator spring operatively connected to the mandrel for biasing the at least one cutter arm and the at least one torque arm to an extended position and a torque arm carrier for pivotal connection to each torque arm. The torque arm carrier is preferably slidingly engaged with the mandrel.

[0014] In other embodiments, the hole initiator system includes at least one initiator finger operable between a retracted and an extended position wherein the at least one initiator finger has dimensions such that in the extended position, the at least one initiator finger is in cutting contact with the borehole or the casing of a borehole. In one embodiment, the at least one initiator finger has a first end extending into the mandrel for enabling actuation of the hole initiator system in response to an initiator tool contacting the first end.

[0015] In other embodiments, it is preferred that the torque arm and the cutter arm carriers include at least one flushing port behind the torque arm.

[0016] In a more specific embodiment, the invention provides an under-reamer tool for creating a large diameter cavity within a well bore comprising: a mandrel for operative connection to a drill pipe, the mandrel operatively supporting a torque arm assembly and a hole initiator assembly, the hole initiator assembly, upon activation, for creating a nominally larger diameter cavity thereby triggering activation of the torque arm assembly, wherein the torque arm assembly includes a cutter arm carrier, a torque arm, a torque arm carrier and an activator spring, the cutter arm carrier in sliding engagement with the mandrel, the torque arm pivotally connected to the cutter arm carrier and to a cutter arm, the torque arm pivotally connected to the mandrel and the activator spring operatively connected to the mandrel for biasing the at least one cutter arm and the at least one torque arm to an extended position.

[0017] In another embodiment, the invention provides a method of under-reaming a well bore comprising the steps of:

[0018] advancing an under-reamer tool on a drill string as in claim 1 to a location in a well bore;

[0019] activating the at least one initiating fingers;

[0020] advancing the under-reamer tool to create a nominally larger diameter cavity and trigger the torque arm assembly to create a large diameter cavity.

[0021] In a preferred embodiment of the method, it is preferred that the well is reverse circulated during advancement of the under-reamer tool while creating a large diameter cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention is described with reference to the following drawings in which:

[0023]FIG. 1 is a schematic cross-sectional view of the under-reamer in accordance with the invention shown in the retracted position.

[0024]FIG. 2 is a schematic cross-sectional of an under-reamer in accordance with the invention shown in a partially extended position.

[0025]FIG. 3 is a schematic cross-sectional view of an under-reamer in accordance with the invention shown in a fully extended position.

[0026]FIG. 4A is a schematic cross-sectional view of the hole initiation system in accordance with the invention with the bole initiator finger in the closed position.

[0027]FIG. 4B is a schematic cross-sectional view of the hole initiation system in accordance with the invention with the hole initiator finger in the open position.

DETAILED DESCRIPTION OF THE INVENTION

[0028] System Overview

[0029] With reference to the Figures, an under-reamer tool 10 for use in creating large diameter cavities downhole is described.

[0030] With specific reference to FIGS. 2 and 3, the under-reamer 10 is attached to the bottom of a drill pipe 8 to enable the tool 10 to be run downhole. The tool 10 generally includes an upper torque arm system 10 a and a lower hole initiation system 10 b which in combination enable a large diameter cavity to be created downhole by the rotation of the drill pipe. The torque arm system 10 a generally includes a torque arm carrier 12, at least two torque arms 14 (FIGS. 2 and 3 showing a single arm for clarity), a cutter arm 16, a cutter arm carrier 18, and a main activator spring 20. The hole initiation system 10 b generally includes a hole initiator finger 22 that is activated by an initiation tool 58. The torque arm system and hole initiation system are both positioned about a main mandrel 24.

[0031] The main mandrel 24 extends the length of the under-reamer and includes an appropriate attachment system at the upper end for attaching the under-reamer 10 to the drill pipe 8, and at the lower end for attaching a drill bit 30. The mandrel includes appropriate through bores 22 b adjacent the hole initiation system to enable the circulation of drilling fluids between the exterior and interior of the mandrel, thus enabling fluids to circulate between the interior of the drill pipe and the well bore as is known.

[0032] Torque Arm System

[0033] The torque arms 14 and cutting arms 16 are pivotally connected to one another and are pivotally connected to the torque arm carrier 12 and to the cutter arm carrier 18 respectively 16. The cutter arm carrier 18 is in sliding engagement with the mandrel between a cutter arm stop 18 a and the hole initiator system on the mandrel 24. The main activator spring 20 is positioned between the hole initiator assembly 10 b and the cutter arm carrier 18 over the mandrel and generally biases the cutter arm carrier upwardly towards the torque arm carrier 12 for extending the torque arms 14 and cutter arms 16 away from the mandrel 24. FIG. 3 shows the main activator spring in the extended position with the torque arms and cutter arms fully extended. FIGS. 1 and 2 show the main activator spring in fully compressed and partially extended states respectively. As shown, upward motion of the cutter arm carrier 18 is arrested by a cutter carrier stop 18 a on the mandrel 24.

[0034] In a preferred embodiment, the under-reamer 10 includes three torque arms and cutter arms equidistantly spaced from each other (ie at 120 degrees to one another) about the mandrel 24. However, the number of torque arms and cutter arms are primarily limited by the diameter of the tool in a collapsed state that is in turn restricted by the diameter of an existing application's wellbore.

[0035] The torque arm carrier 12 may be fixed to the mandrel. Alternatively, and in a preferred embodiment, the torque arm carrier is in sliding engagement with the mandrel between an annulus pack-off 26 and a torque arm carrier stop 28. A bumper spring 29 for absorbing compression loads biases the torque arm carrier 12 downwardly. The annulus pack-off preferably includes shear pins 26 a to enable removal of the drill pipe in the event that the tool becomes jammed downhole.

[0036] The torque arms 14 are dimensioned to extend the cutter arms outwardly at an appropriate cutting angle and to collectively define the maximum diameter of the under-reamed cavity of the bore hole. In addition, the torque arms must be of adequate strength to effectively transfer the torque load of the rotating drill string to the cutter arms 16.

[0037] The cutter arms 16 are dimensioned in conjunction with the torque arms 14 such that they engage the borehole at an appropriate cutting angle that enhances the ability of the circulating medium to clear the cuttings away from the cutting surface under the force of gravity. In addition, the cutter arms are provided with cutting surfaces or teeth that preferably assist in the extension of the tool by the design of the cutting surfaces.

[0038] That is, the cutting surfaces of the cutter arms can include a variety of designs that may be selected on the basis of the rock type and its related fracturing characteristics. Generally, it is advantageous to match the cutting surface design to the rock type such that the resulting rock cuttings are sufficiently small enough to enable them to be circulated away from the cutting area, but large enough to enable the cavity to be milled as fast as prudently possible while also enabling the circulation fluids to evacuate cuttings from the cutting face and the cavity with reasonable circulation speeds.

[0039] Hole Initiator System

[0040] The hole initiator system 10 b includes at least one hole initiator finger 22 operable between a retracted position where the hole initiator finger does not interfere with the passage of the tool through the well bore (cased or uncased) and an extended position where the hole initiator finger makes contact and progressively engages the well bore or casing until it is fully extended whereby rotation of the tool cuts away the casing or well bore wall to a nominally larger diameter. This nominally larger diameter then permits the torque arms 14 to extend as will be explained in greater detail below.

[0041] More specifically and with reference to FIGS. 4A and 4B, the hole initiator system 10 b includes a main body 50 for operative connection to the lower end of the mandrel and at least one hole initiator finger 22 pivotally mounted to the main body 50. For clarity, a single hole initiator finger 22 is shown in FIGS. 4A and 4B although it is understood that a number of fingers (typically three) may be mounted within the main body 50. The main body further supports an internal spring 52 for applying an upward biasing force against aninitiator ring 54. The initiator ring is moveable between upper and lower positions wherein movement of the initiator ring from the upper position to the lower position against the biasing force of the internal spring 52 triggers the outward extension of the hole initiator finger 22 in conjunction with leaf spring 56.

[0042] Leaf spring 56 is attached to the main body 50 and is slidable between an upper position as shown in FIG. 4A and a lower position as shown in FIG. 4B. Leaf spring 56 is outwardly biased with respect to the main body 50 and is retained on the main body by cap screw 56 a. In the upper position, the leaf spring 56 contacts the hole initiator finger at a position above the pivot axis 22 b of the hole initiator finger 22 thereby causing the outer tip 22 a of the hole initiator finger 22 to be inwardly biased and thus retained in the retracted position. Upon movement of the initiator ring 54 to the lower position (as shown in FIG. 4B), the initiator ring 54 contacts a heel portion 22 c of the initiator finger 22 causing the initiator finger 22 to pivot outwardly while simultaneously pushing the leaf spring 56 to the lower position. As the leaf spring 56 moves to the second position, the leaf spring 56 contacts the initiator finger 22 at a position below the pivot axis 22 b of the hole initiator finger 22 thereby applying an outwardly biasing force against the initiator finger 22.

[0043] As shown in FIG. 4B, once in the extended position, the initiator ring 54 prevents inward movement of the initiator finger 22 by contact with the heel 22 c of the initiator finger 22. Furthermore, in the event that initiator ring 54 is moved upward out of position, the leaf spring supports the initiator finger in the extended position.

[0044] The hole initiation system 10 b is activated downhole by pumping an initiation tool 58 through the mandrel and against the initiation ring 54 which causes the initiation ring to move from the upper to the lower position. As shown, the initiation tool 58 is hollow and includes a small orifice 58 a that allows circulation of the assembly during activation and that also enables a pressure differential to be established between the initiation tool 58 and the initiator ring 54 to enable movement of the initiator ring 54 with respect to the main body 50. After the hole initiation system has been activated, and the well is reverse circulated, the initiation tool 58 will be pumped to surface where it is retrieved.

[0045] Operation

[0046] The complete operation of the tool is explained with reference to FIGS. 1-3.

[0047]FIG. 1 shows the under-reamer tool in an initial collapsed state within drill pipe casing in a well bore. From within the casing the under-reamer tool is run down the cased or uncased wellbore to the area to be under-reamed. The well is then circulated in the normal manner by pumping fluid through the main mandrel of the under-reamer and out the bottom of the tool to the annulus between the tool and the drill pipe casing 9 or uncased wellbore.

[0048] The initiation tool 58 is then inserted into the drillpipe and pumped through the main mandrel of the under-reamer whereby the hole initiation system 10 b is activated as described above.

[0049] As the drill string is then rotated and advanced, the hole initiator fingers 22 cut a nominally larger diameter cylindrical cavity through the casing 8 or well bore surface thereby creating a lip or edge 11 at the location where the hole initiator fingers 22 first begin to extend. As the torque arm/torque arm carrier pivot point 12 a advances past the lip or edge, this allows the cutter arm and torque arm to advance radially as the tool is advanced downhole. As the lip is encountered, the cutter arm begins to extend under the compressive force of the main activator spring 20 and reverse circulation is normally commenced as the cuter arms begin to extend. As the torque arm 14 continues to advance past the lip 11 (FIG. 2), the cutting surfaces of the cutter arm make contact with the formation and will extend under the action of the main initiator spring 20. The tooth design on the cutter arms may also tend to advance the cutter arms to the extended position.

[0050] It is preferred that the torque arm includes a wear guide that extends outwardly and prevents the cutter arm and the cutter arm teeth from making contact with the inside surface of the wellbore or casing when in the collapsed position. The wear guide thereby helps to ensure that the tool can be run into the cased or uncased well bore such that no appreciable wear occurs on the cutter teeth as well as to prevent damage to the casing of a well.

[0051] Fluid Circulation

[0052]FIG. 3 illustrates the under-reamer tool in the fully extended position. In the fully extended position, the main activator spring 20 is fully extended to position the torque arm 14 and the cutter arm 16 in the extended position shown in the Figure. During the under-reaming process, fluid circulation is reversed, whereby fluid is pumped through the annulus of the drill pipe casing and up through the main mandrel of the under-reamer tool. Initially, this enables the retrieval of the initiator tool 58. Another key result of reverse circulation is assisting cutting debris to fall down the sloped edges of the cutting face at the bottom of the cavity. As noted above, it is preferred that reverse circulation is commenced as the cutter arms begin to extend.

[0053] Preferably, a tri-cone bit is connected to the bit box 32 at the lower end of the mandrel 24 will crush any oversized cutting debris particles to prevent clogging of the circulation orifices through the tool and to allow them to be properly carried uphole with the circulation medium.

[0054] Under-Reamer Extraction

[0055] To retract and extract the under-reamer tool after the desired cavity size has been made, rotation of the under-reamer tool is halted. The circulation flow direction is returned to normal (i.e. down the drill pipe through the main mandrel 24 and up the wellbore annulus) to enable flushing of the cutter arms, torque arms and main mandrel 24 of any debris that may have lodged on any of these surfaces which thereby assists inensuring that the cutter arms 12 and the torque arms 14 will properly collapse to enable withdrawal of the tool. In this regard, it is noted that washing ports may be provided at appropriate locations along the mandrel (specifically at the pivot points 12 a, 18 b) to enhance the removal of any debris from the torque arm carrier or cutter arm carrier that would prevent the torque arms and cutter arms from properly retracting. In order to provide increased hydraulic pressure at the washing ports, a ball may be placed within the mandrel that seals against the initiator ring 54.

[0056] After the well has been flushed, the under-reamer tool is pulled up through the upper cased or uncased wellbore. As the mandrel is raised, the torque arms 14 contact the upper surface of the cavity 11 a and thereby force the torque arm and the cutter arm to the collapsed position. As the torque arms are contacting the upper surface of the cavity 11 a, the cutter arms will exert a downward force on the main activator spring and thereby compress the main activator spring enabling both the torque arms and the cutter arms to withdraw against the mandrel. As the hole initiator fingers 22 contact the cased or uncased well bore, the fingers will also retract against the outwardly biasing force of the leaf spring 56.

[0057] As the torque arms, cutter arms and initiator fingers become fully withdrawn, the tool may be withdrawn up the well bore.

[0058] At the surface, the initiator fingers 22 may be reset to the retracted position by adjusting the leaf spring 56 to the upper position.

[0059] The under-reamer tool is particularly advantaged over past tools by the relative size and geometry of the torque arm and cutting arm, and the ability to efficiently circulate the large volume of cuttings away from the cutting surface and ultimately evacuate the cuttings from the large diameter cavity. As a result, it is capable of boring large diameter cavities due to the length and integral torque strength of the retractable cutter arm assembly sufficient to transfer torque of the main mandrel to the cutting face of the cutter arm. In particular, a torque arm that extends perpendicularly and pivotally supports a longer cutter arm has been shown to be particularly effective in achieving larger diameter cavities in the field.

[0060] It is also preferred that the relative geometries of the fully extended torque arm and cutter arm relative to the mandrel define an inwardly tapering cone of sufficient steepness (ie greater than the angle of repose for the rock materials and typical particle sizes) such that cutting debris will fall to the bottom of the cone without requiring fluid circulation.

[0061] Furthermore, the combination of the main activator spring, cutter arm carrier, cutter arm, torque arm and torque arm carrier enables automatic engagement or extension of the cutter arm as the under-reamer tool is advanced. Retraction of the under-reamer tool is also automatic as the torque arm is forced to a collapsed position upon as the tool is retracted up the wellbore.

[0062] In operation, the tool can provide in the order of 4000 ft-lbs of torque at typical milling speeds of 28-96 rpm. Reverse circulation velocities of 200 ft/min can be achieved with liquid circulation fluids.

[0063] The tool may also be effectively used in a gas well. In a gas well, where a gas is used as the circulation fluid, the circulation velocity will be increased to a minimum of approximately 600 ft/min and preferably 1000 ft/min. In addition, activation of the hole initiation system 10 b will require an initiation tool 58 having a smaller orifice diameter 58 a to enable sufficient hydraulic pressure to be established to move the initiator ring 54 against spring 52. 

What is claimed is:
 1. An under-reamer tool for creating a large diameter cavity within a well bore comprising: a mandrel for operative connection to a drill pipe, the mandrel operatively supporting a torque arm assembly and hole initiator assembly, the hole initiator assembly, upon activation, for creating a nominally larger diameter cavity thereby triggering activation of the torque arm assembly.
 2. An under-reamer tool as in claim 1 wherein the mandrel and hole initiator assembly are hollow enabling reverse circulation of the well bore after activation of the hole initiator assembly.
 3. An under-reamer tool as in claim 1 wherein the torque arm assembly includes a cutter arm carrier in sliding engagement with the mandrel.
 4. An under reamer tool as in claim 1 wherein the torque arm assembly includes at least one cutter arm and at least one torque arm, wherein each cutter arm is pivotally connected to the cutter arm carrier and to a torque arm and wherein each torque arm is pivotally connected to the mandrel.
 5. An under-reamer tool as in claim 1 wherein the torque arm assembly includes an activator spring operatively connected to the mandrel for biasing the at least one cutter arm and the at least one torque arm to an extended position.
 6. An under-reamer tool as in claim wherein the torque arm assembly includes a torque arm carrier for pivotal connection to each torque arm.
 7. An under-reamer tool as in clam 6 wherein the torque arm carrier is slidingly engaged with the mandrel.
 8. An under-reamer tool as in claim 1 wherein the hole initiator system includes at least one initiator finger operable between a retracted and an extended position wherein the at least one initiator finger has dimensions such that in the extended position, the at least one initiator finger is in cutting contact with the borehole or the casing of a borehole.
 9. An under-reamer tool as in claim 8 wherein the at least one initiator finger has a first end extending into the mandrel for enabling actuation of the hole initiator system in response to an initiator tool contacting the first end.
 10. An under-reamer as in claim 1 further comprising a drill bit in operative connection with the mandrel for crushing oversized debris cuttings that are milled from a formation during an under-reaming operation.
 11. An under-reamer tool as in claim 3 wherein the torque arm carrier includes at least one flushing port behind the torque arm.
 12. An under-reamer tool as in claim 6 wherein the cutter arm carrier includes at least one flushing port behind the cutter arm
 13. An under-reamer tool for creating a large diameter cavity within a well bore comprising: a mandrel for operative connection to a drill pipe, the mandrel operatively supporting a torque arm assembly and a hole initiator assembly, the hole initiator assembly, upon activation, for creating a nominally larger diameter cavity thereby triggering activation of the torque arm assembly, wherein the torque arm assembly includes a cutter arm carrier, a torque arm, a torque arm carrier and an activator spring, the cutter arm carrier in sliding engagement with the mandrel, the torque arm pivotally connected to the cutter arm carrier and to a cutter arm, the torque arm pivotally connected to the mandrel and the activator spring operatively connected to the mandrel for biasing the at least one cutter arm and the at least one torque arm to an extended position.
 14. An under-reamer tool as in claim 13 wherein the hole initiator system includes at least one initiator finger operable between a retracted and an extended position wherein the at least one initiator finger has dimensions such that in the extended position, the at least one initiator finger is cutting contact with the borehole or the casing of a borehole.
 15. An under-reamer tool as in claim 14 wherein the at least one initiator finger has a first end extending into the mandrel for enabling actuation of the hole initiator system in response to an initiator tool contacting the first end.
 16. A method of under-reaming a well bore comprising the steps of: advancing an under-reamer tool on a drill string as in claim 1 to a location in a well bore; activating the at least one initiating fingers; advancing the under-reamer tool to create a nominally larger diameter cavity and trigger the torque arm assembly to create a large diameter cavity.
 17. A method as in claim 16 further comprising withdrawing the drill pipe to retract and extract the under-reamer.
 18. A method as in claim 16 wherein the well bore is reverse circulated during advancement of the under-reamer tool while creating a large diameter cavity.
 19. A method as in claim 16 wherein the cutter arms are interconnected to ensure simultaneous extension and retraction.
 20. A method as in claim 16 wherein the cutter arms are allowed to cut a tapered hole that ensures that the cuttings are preferentially travel to the bottom of the well bore to be further evacuated by reverse circulation.
 21. A method as in claim 16 wherein the cutter arms are allowed to cuta tapered hole that ensures that the cuttings are preferentially travel to the bottom of the well bore and any oversized cutting debris to be reduced in size by a typical tri-cone bit to better enable all the cutting debris to be crushed to a size that can be the further evacuated by reverse circulation.
 22. An under-reamer tool as in claim 1 wherein the under-reamer tool includes a drill pipe connector connected to the main mandrel with non-torque transmitting shear pins.
 23. An under-reamer tool as in claim 1 wherein the under-reamer tool includes a drill pipe connector slidingly connected to the main mandrel, the drill pipe connector having a bumper spring between the drill pipe connector and the torque arm carrier for enabling controlled torque relief for the cutter arm assembly. 